"Tom Anderson" wrote in message
h.li...

An intermediate possibility is for HEx to be extended to Staines.

This is the only reason i can think of for doing it. Even then, it means
sacrificing the ability to run third-rail stock into T5 for this mythical
future option.

As the consultation report points out though, the two dedicated Airtrack
platforms under T5 are a terminus from the west, it is only the HEx
platforms that can be turned into through platforms, joining Airtrack
somewhere to the west of the station. So even extensions to HEx might'nt
necessarily affect terminating DC trains if the changeover was in the
vicinity of the station.

Is it the case that existing stock won't be allowed anyway, for safety or
ABB [1] reasons?

If existing stock such as 450s was used by SWT - except for having shoes
rather than pantograph it is to all intents identical to the Heathrow
Connect 360s [ref other discussions about AC traction in all modern EMUs],
with the end gangways being an additional safety feature, [although not
required in the Heathrow tunnels as they have walkways].

I also don't believe there are any increased safety issues with underground
third rail stations, if there was such a huge issue why would they be
carrying on with a new third rail installation for the ELL? Another poster
has mentioned corrosion in the tunnels, but this is surely overcome with
modern slab track and fastenings in concrete linings - they don't suffer
from groundwater ingress like the early LU tunnels...

Paul S